1. Field of the Invention
The present invention relates in general to an upper support used in a suspension system of a motor vehicle, for elastically connecting a body of the vehicle and a shock absorber of the vehicle, and more particularly to such an upper support which has improved vibration-isolating capability while assuring high running and steering stability of the vehicle.
2. Discussion of the Prior Art
In a conventional suspension system of a motor vehicle, a generally cylindrical upper support is usually interposed between a body of the vehicle and a piston rod of a shock absorber which is linked with an axle or arm for supporting vehicle wheels. The upper support is adapted to prevent input vibrations received from the wheels through the shock absorber from being transmitted to the vehicle body, for example. Examples of such an upper support are disclosed in laid-open Publication Nos. 55-24519 and 55-24520 of unexamined Japanese Utility Model Applications. The upper support as disclosed in these publications generally consists of a cylindrical inner rigid member to which the piston rod of the shock absorber is fixed, a cylindrical outer rigid member which is disposed radially outwardly of the inner rigid member and fixed to the vehicle body, and an elastic body interposed between the inner and outer rigid members for elastically connecting the two rigid members.
The upper support as described above is required to be given a soft spring characteristic for improved vibration-isolating capability, for the purpose of preventing transmission of the vibrations from the shock absorber toward the vehicle body. At the same time, the upper support is required to exhibit a high degree of stiffness so as to minimize the amount of deformation thereof due to a change in the static load upon turning, braking or rapid acceleration or deceleration of the vehicle, for the purposes of avoiding excessive changes in the attitude of the vehicle, and thus assuring high running and steering stability of the vehicle.
To meet the above requirements of the upper support, the dynamic spring characteristic is adjusted to enhance the vibration-isolating capability, while the static spring characteristic is adjusted to enhance the running stability of the vehicle.
However, it is extremely difficult for the known upper support to fully satisfy these requirements for the vibration-isolating capability and the vehicle running stability, since the known upper support relies only upon elastic deformation of the elastic body for isolating the input vibrations. Namely, since there is a certain correlation between the dynamic and static spring characteristics of the elastic body, it is impossible for the upper support to provide a soft dynamic spring characteristic without exhibiting a soft static spring characteristic. Further, since the upper support used for the suspension system, in particular, is likely to receive a relatively large static load during use, the elastic body must be made of a rubber material having a relatively high static spring constant, which inevitably causes stiffening of the dynamic spring characteristic of the upper support. Although the upper support is required to isolate relatively high-frequency vibrations, such as road-oriented noises, which have frequencies of several tens to hundreds of hertz, the dynamic spring constant of the elastic body of the upper support tends to unfavorably increase with an increase in the frequency of the input dynamic vibrations. Thus, the known upper support suffers from an extremely high dynamic spring constant when it is subjected to the middle- to high-frequency vibrations as described above.
In the suspension system using the upper support as described above, it is also noted that the shock absorber may be oscillated due to lateral movements of the suspension arm or other components connected thereto. The oscillation of the shock absorber causes the inner rigid member to be pivotally displaced relative to the outer rigid member in a plane including the axes of the inner and outer rigid members, such that the upper and lower ends of the inner rigid member are moved in the radially opposite directions. The conventional upper support is not adapted to provide a spring characteristic suitable for dealing with the pivotal displacement of the inner rigid member relative to the outer rigid member as described above, but is likely to exhibit a relatively high dynamic spring constant or stiffness with respect to the shakes of the shock absorber applied to the upper support, resulting in lowered smoothness of movements of the suspension arm, and deterioration in the driving comfort of the vehicle.